Required Practical 1: Titrations

Method for measuring mass accurately

1. Measure the mass of weighing bottle with the required quantity solid in it on a 2d.p or 3d.p balance

2. Empty the solid into a flask etc. (wtvr ur using in the experiment)

3. Reweigh the now empty weighing bottle

4. Subtract the mass of the first reading from the mass of the empty weighing bottle to give the exact mass of solid actually added

Method for diluting a solution up to 250cm³

1. Pipette 25.0cm³ of original solution into a 250cm³ volumetric flask

2. Make up to the 250cm³ mark with distilled water, using a dropping pipette for the last few drops

3. Invert flask several times to ensure uniform solution

Advantage of using a volumetric pipette over a measuring cyclinder

Volumetric pipette is more accurate as it has a smaller uncertainty

Give one reason why you shouldn’t heat or put hot solutions in a volumetric flask

The heat would cause the flask to expand, so the volume would then be incorrect

Method on how to make a 250cm³ solution

1. Weigh the sample bottle containing the solid on a balance

2. Transfer the solid to beaker and reweigh the now empty sample bottle

3. Record the difference in mass

4. Add distilled/deionised water to the beaker and stir with a glass rod until all solid has been dissolved

5. Transfer this solution to a volumetric flask using a funnel

6. Rinse the beaker and funnel with distilled/deionised water

   • add the washings from the beaker and glass rod to the volumetric flask

7. make up to 250cm³ with distilled/deionised water and then invert the flask several times to ensure uniform concentration throughout the solution

How to make up a volumetric solution and carry out a simple acid-base titration

1. Rinse equipment

   • burette with acid, pipette with alkali and conical flask with distilled water

2. Pipette 25 cm³ of alkali into conical flask

3. Touch surface of alkali with pipette to ensure correct amount is added

4. Add acid solution from burette

   • make sure the jet space in the burette is filled with acid

5. Add a few drops of indicator and refer to colour change at end point

6. Place a white tile underneath the flask to help observe the colour change

7. Add acid to alkali whilst swirling the mixture and

   • add acid drop wise near the end point

8. note burette reading before and after addition of acid

9. repeats titration until at least 2 concordant results are obtained

Why is conical flask used in preference to a beaker?

Because its easier to swirl the mixture in a conical flask without spilling the contents

What would happen if the jet space was not filled properly?

would lead to errors if it fills during the titration, leading to a larger than expected titre reading

Why should u remove the funnel from the burette?

• because small drops of liquid may fall from the funnel into the burette during titration

• leading to a false burette reading

  • would give a lower titre volume

what are indicators and why should you not add too much of them?

• Indicators are generally weak acids so only add a few drops of them

• If too much is added they will affect the titration result

Why may distilled water be added to the conical flask during a titration?

To wash the sides of the flask so that all the acid on the side is washed into the reaction mixture in order to react with the alkali

• adding water does not affect titration reading as the water doesn’t react with reagents, or change the number of moles of acid added

What can we say lf 2 or 3 values are within 0.10cm³ and therefore concordant

We can say

• results are accurate and repeatable

• titration technique is good and consistent

How should titration results be recorded?

• Results should be clearly recorded in a table

• Result should be recorded in full

  • i.e. both initial and final readings

• Record titre volumes to 2d.p 0.05 cm³